Natural gas pipelines from a natural gas reservoir of from a gas producing platform often contain significant amounts of liquids, such as hydrocarbon condensations and water, and optionally smaller amounts of solids that may interfere with the proper operation of downstream equipment and processes. The liquids in a natural gas pipeline are often referred to as ‘slugs’. In order to separate the liquids and solids from the natural gas, fluids and solids from a natural gas pipeline are typically received into a slug catcher. The slug catcher is normally sized so that it is large enough to receive the volume of liquids and any solids when pigging operations are being performed or have recently been performed. The slug catcher is also designed to separate the fluids received and particularly to separate most of the gas from the liquid and solids and also preferably to separate the liquids into to a liquid water phase and a liquid hydrocarbon phase.
Although generally satisfactory, slug catcher's efficacy in such separation can be compromised in case of a surge, i.e. when an unexpectedly large slug of liquids and optionally solids is deposited in the slug catcher. This can for example result in downstream equipment, such as a three-phase separator, being starved of gaseous hydrocarbons which may cause shut-down of further downstream processing units, e.g. an LNG plant, a Fischer-Tropsch plant or a methanol plant. It may also result in the supply of liquids to a downstream gas processing system.
Typically a surge is caused by the operation of pipeline pigs in the pipeline. Such pigs are used for periodic cleaning of the pipeline or for other operations. In certain operations such as for batch inhibition, a liquid slug is provided between two pipeline pigs. A single pig may be used for cleaning and in use, a large slug will gather in front of the pig. This large slug will cause a surge when it enters the slug catcher.
In the prior art, several methods have been disclosed to deal with unexpectedly large slugs in natural gas pipelines. Reference is made in this respect for example to EP 331 295 A and U.S. Pat. No. 4,160,652.